New aspects of superconductors

Researchers have uncovered new aspects of superconductivity by studying films of niobium nitride measuring only a few tens of nanometres thick.

In contrast with previous theory, the researchers claim that weakly bound electrons in the superconducting system continue to exist even after the zero-resistance state is destroyed. These experiments provide a new perspective on the physics of high-temperature superconductors.

In a superconductor, the 'super-current' is carried by weakly bound pairs of electrons, called Cooper pairs. The formation of these pairs induces a gap in the electronic energy spectrum, which appears in conventional superconductors at the same temperature where zero-resistance appears.

However, recent experiments have indicated that even after global superconductivity is destroyed, the material continues to possess some of the fundamental properties commonly associated with the superconducting state. To explore this aspect experimentally, the researchers grew films of niobium nitride on magnesium oxide substrate.

Using scanning tunnelling spectroscopy measurements on strongly disordered niobium nitride thin films, the study shows that the gap in electronic spectrum associated with superconductivity persists up to temperatures many times higher than the temperature at which the resistance reaches 1% of its normal state value. This suggests that Cooper pairs continue to exist in the system even after the zero-resistance state is destroyed.

"The main implication of our work is that it provides new insight into the possible origins of the pseudogap phase, an energy range in superconductivity that is considered central to the physics of high-temperature superconductors," says lead researcher Pratap Raychaudhuri.

The authors of this work are from: Tata Institute of
Fundamental Research, Mumbai, India and ISC-CNR and Department of
Physics, Sapienza University, Piazzale Aldo Moro, Rome, Italy.